Conventional watermarks, such as are produced by the pressure of a projecting design, as in the mold, during the manufacturing process of paper are well known. The watermark can be seen when the paper is held up and back-lit by show-through illumination. The watermark is intended not to be visible when the paper is not held up, e.g., at rest on a desk and front-lit by incident illumination. Watermarks are useful for authentication purposes or identification of an origin or author of the resident document.
Much of the printing in today's business and scientific world is accomplished electronically, such as with digital color printer engines, which inherently provide only a limited number of output possibilities due to their binary nature, i.e., they produce either a dot or no dot at a given pixel location. Pressure formed watermarks can only be incorporated into the original paper stock in such printers and cannot be inserted at the time of printing.
The well known notion of halftoning (representing a continuous tone image with a binary representation) can produce an image including rapidly varying spatial patterns normally not visible to a human visual system, but artifacts thereof can be, especially those generated by interaction between adjacent or overlaying printing patterns.
A number of different techniques may be used for generating a halftone image from a continuous tone representation [Note R. Ulichney, “Digital Halftoning”, MIT Press, Cambridge, Mass., 1987 and J. P. Allebach, Ed., “Selected Papers on Digital Halftoning”, SPIE, Bellingham, Wash., and G. Sharma, “Digital Color Imaging”, IEEE Transactions on Image Processing, Vol. 6, No. 7, July 1997, pp. 901-932]. In one commonly used technique, over a given image area having a number of contone pixels therein, each pixel value of an array of pixels is compared to one of a set of preselected thresholds and a decision is made whether to produce a dot or no dot at the pixel location in the output binary halftone based on the result of the comparison (the thresholds may be stored as a dither matrix and the repetitive pattern generated by this matrix is considered a halftone cell) as taught for example in U.S. Pat. No. 4,149,194 to Holladay.
The dither matrix of threshold values is often referred to as a “screen”, and the process of generating the binary image from the contone image using the screen is called “screening” or dithering. Conventional digital halftones start as a number of isolated dots that grow bigger as more colorant is requested on the paper. These screens are referred to as clustered-dot screens. The fundamental rate at which the dots in a clustered dot screen are repeated is commonly referred to as the screen's spatial frequency [Note, R. Ulichney, “Digital Halftoning”, id.]. The higher the screen spatial frequency, the finer and smoother appearing the image and also the greater is the capacity for the dots to represent fine detail in the image.
Stochastic or non-periodic screening is an alternative to conventional clustered dot screens. Instead of producing dots that grow with increased colorant on paper, the stochastic screening method produces a well-dispersed pattern of isolated dots at spaced pixel locations. Thus there is no fundamental periodicity in the dots, instead the design of the screen attempts to produce patterns with pleasant noise characteristics. The pleasant noise characteristics are achieved by designing the screen so as to distribute the noise energy in the region of high spatial frequency, where the human visual system has a significantly reduced sensitivity. In this respect, U.S. Pat. No. 5,673,121 to Wang, discloses a stochastic halftone screening method for designing an idealized stochastic screen. Watermarking documents through the introduction of correlations in adjacent regions of a stochastic screen in the screen design process is disclosed in U.S. Pat. No. 5,734,752 to Knox and U.S. Pat. No. 5,790,703 to Wang.
A system utilizing halftone outputs produced by stochastically clustered pixel screens, referred to as “stoclustic” screens (U.S. Pat. No. 5,859,955 to Wang) combines the advantages of both cluster halftone and stochastic halftone screens. The use of stoclustic halftone screens for watermarking of images through the use of suitable halftones is also known and disclosed in related U.S. application Ser. No. 09/069,095 to Wang and is incorporated herein by reference as it discloses a particular clustered-dot screen design method useful in implementation of the subject invention, as will be more fully explained below. But this method requires that the watermark detection is performed by either electronic scanning and image processing of the printed image or by overlaying a distinct transparency on a printed image containing the watermark for effectively interacting the distinct screen transparency with the printed image. Watermark identification requiring scanning and additional image processing or via a separate transparency document is inconvenient and undesirable. Such problems are intended to be overcome by the present invention.
Duplex printing is fairly common for increased paper use efficiency, and for the subject invention is exploited for the selective interaction of images on both sides of the document to effectively achieve an overlaying of patterns forming a desired watermark upon show-through illumination.